It is now common practice to explore the oceans of the earth for deposits of oil, gas and other valuable minerals by seismic techniques in which an exploration vessel imparts an acoustic wave into the water, typically by use of a compressed air "gun." The acoustic wave travels downwardly into the sea bed and is reflected at the interfaces between layers of materials having varying acoustic impedances. The wave travels back upwardly where it is detected by microphone or "hydrophone" elements in a streamer towed by the vessel to yield information regarding characteristics of the underwater material and structures.
A towed streamer comprises a plurality of pressure-sensitive hydrophone elements enclosed within a waterproof jacket and electrically coupled to recording equipment onboard the vessel. Each hydrophone element within the streamer is designed to convert the mechanical energy present in pressure variations surrounding the hydrophone element into electrical signals. Due to its extreme length (on the order of miles), the streamer is often divided into a number of separate sections or "modules" that can be decoupled from one another and that are individually waterproof. Individual streamers can be towed in parallel through the use of paravanes to create a two dimensional array of hydrophone elements. Data buses running through each of the modules in the streamer carry the signals from the hydrophone elements to the recording equipment (so-called "acoustic data").
In addition to acoustic data, it is also important to collect and transmit data concerning operational status of the array to the vessel (so-called "nonacoustic data"). Nonacoustic data comprises physical characteristics of interest regarding the operation of each module, including whether water has invaded a module in the streamer, module temperature, module depth and power supply voltage.
It is not uncommon for individual modules to fail during operation of the streamer or during storage or deployment. Failure can occur due to saltwater invasion, shorting electronic components within the streamer. The electronic components can fail by overheating, handling excess power or simply by chance. Mechanical flexing and compressing of the streamer can cause damage, too.
Today, many towed arrays have digital data channels. With digital data transmission, data transmission rates are higher and, with proper attention to electromagnetic interference, data fidelity is maintained from the hydrophone to the recording equipment.
For instance, U.S. Pat. No. 3,996,553, that issued on Dec. 7, 1976 is directed to a plurality of data acquisition units connected to a central signal processor through a common digital telemeter link. The telemeter link includes a data channel, an interrogation channel and a control channel. The central signal processor sends an interrogation signal through the interrogation channel to the data acquisition units. As each data acquisition unit recognizes the interrogation signal, it transmits its acquired data back up to the central processor through the data channel. Any selected data acquisition unit, when it receives a control signal through the control channel at the same time that it receives an interrogation signal through the interrogation channel, can be caused to perform a function different from all other units. The signal propagation velocity through the control channel is different from the signal propagation velocity through the interrogation channel. One of the two signals may be transmitted through the faster channel at a selected time later than the other of the two signals is transmitted through the slower channel. The selected time difference between the transmission of the two signals is proportional to the ratio of signal propagation delay difference between channels. Accordingly, the signal propagating through the faster channel will overtake and intercept the signal propagating through the slower channel at the selected data acquisition unit. Each data acquisition unit may have one or more input channels. Each input channel is connected in turn to the data channel through a stepping switch or multiplexer. The interrogation signal may exist in one of two or more states. In the first state, the interrogation signal resets the multiplexer, in the second state, the interrogation signal advances the multiplexer to the next input channel in sequence. Thus, this system allows for control signals to command changes of state within the individual multiplexers in the system.
Unfortunately, if the central processor fails to generate the interrogation and control signals properly, the streamer ceases operation, unable to synchronize transmission of data from the data acquisition units. Even if the central processor is functioning, damage to the interrogation or control channels can render the streamer useless.
It is preferable to have a central or master clock source for generating timing signals in a streamer to allow synchronous transmission of data. In some streamers, the master clock source is located in an aftmost module of the streamer. The master clock signal is generated in the aftmost module and transmitted forward, circuitry in more forward modules slaving to the master clock signal to synchronize their operation. However, should the master clock source fail, it is highly desirable to provide an alternative source to assume the role of master source to allow at least a portion of the streamer to continue operation. The prior art has failed to provide for operation of only a portion of a streamer in case a malfunction renders part of the streamer inoperative.